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Elusive Sour Taste Cells Revealed

Humans can taste many different compounds, but they fall into five basic groups: sweet, bitter, sour, salty and umami (the taste of monosodium glutamate). The cells that detect sweet, bitter and umami have already been identified. Now, the same research group that pinpointed those has identified the long-sought mammalian taste cells that serve as our sour sensor.

Sweet,
bitter and umami tastes are detected by distinct receptor proteins, each found
in independent populations of taste receptor cells (TRC) on the tongue. A group
of researchers led by Dr. Nicholas Ryba at NIH's National Institute of Dental
and Craniofacial Research and Dr. Charles S. Zuker, a Howard Hughes Medical Institute
investigator at the University of California at San Diego, set out to identify
the mammalian sour taste sensor (NIH's National Institute on Deafness and Other
Communication Disorders also contributed funding to the study). Since these proteins
must be found on the outside of TRCs, the researchers searched the mouse genome
for proteins that spanned the outer membranes of these cells.

In the August 24, 2006 issue of Nature, the team reports their discovery
of a protein called PKD2L1 that's found in a subset of TRCs distinct from those
responsible for sweet, bitter and umami taste. The researchers genetically engineered
mice with a toxin so that the TRCs producing PKD2L1 are killed. The mice without
these TRCs didn't respond to sour stimuli, but their responses to other tastes
remained.

When something tastes sour to you, you're essentially sensing how acidic it
is. Acid sensing is important not only in the taste system, but for monitoring
body fluids. For example, carbon dioxide levels can change the pH (a measure
of acidity) of the blood and cerebrospinal fluid. The researchers thus looked
to see whether PKD2L1 might be found in other cell types, and whether these cells
could sense changes in acidity.

They found PKD2L1 in a type of nerve cell surrounding the central canal of the
spinal cord. Further testing showed that these cells can send out signals in
response to changes in pH. The researchers propose that these nerve cells may
function as sensors of cerebrospinal pH. PKD2L1 thus may play a role in acid
sensing in both types of cells.

This research shows that the unique TRCs producing PKD2L1 are the long-sought
sour taste detection cells. Exactly how these cells detect sourness and what
PKD2L1's role in the process might be are subjects for further research. In addition,
the researchers are now searching for the final taste receptor cells, those for
salty tastes.